1. Field of the Invention
The present invention relates to a high thermal conductivity battery assembly; in particular, it is suitable to serve as building block for drive train battery system for electric vehicles, solar energy storage, and grid storage applications.
2. Description of Related Art
The invention of high energy density battery technologies initiated the market of cellular phones, notebook computers, electric cars, and grid storage applications. The market demand for smaller form factor and longer operating hours pushing the battery makers to invent higher and higher energy density battery cells. Higher energy density cells require more chemically active materials and inherently less stable and more difficult to design battery systems with high safety factor.
Most high energy density battery cells are consumed in cellular phones and portable computers. Total energy required for these devices are small, and relatively few battery cells, up to 8 of the 18650 form factor, are required. Battery life expectation for consumer products is typically 1-2 years. It is relatively easy to design a safe product with only a few battery cells.
However, battery pack for electric vehicles requires a lot of battery cells. A small electric car with 21 KW-hour capacity can have a driving range of 100 Km, and would require 3,000 of the 18650 form factor battery cells, each with 7 watt-hour capacity. By necessity these battery cells must be packaged tightly together with only millimeter spacing, and could generate about 1,000-watt heat load while in operation. Without careful thermal design, battery cell temperature could elevate up to 30 Celsius above ambient, with detrimental effect on battery cell life. Battery system for grid storage and vehicle operation is often 5 years or more, thermal management is a tough design issue for vehicle applications. Furthermore, high energy density battery cells become unstable when internal temperature exceeds 80 degree Celsius. It is cell chemistry and process dependent, and the probability of thermal run away, cell venting, and fire and explosion increases dramatically beyond such safe temperature limit, and the battery pack design must not exceed this limit.
When one battery cell goes into thermal run away, either through violation of safe temperature limit, manufacturing process induced cell short circuit, over charge, or external impact from vehicle crashes, the amount of energy released may cause adjacent battery cells to also go into thermal run away, this chain reaction destroys the battery pack and place the vehicle passengers in great physical danger. Therefore it has been one of major research subjects for the industry to effectively disperse such generated heat and to prevent thermal runaway.
To resolve battery life and safety issues, one common heat dissipation design can be shown in FIG. 1, wherein cooling fins for heat sinking are added to the sides of each battery cell 2 and hence gaps can be created between such battery cells 2 thereby, using forced air convections through air blow generated by the fan 20, it is possible to carry away heat energy from the edges of battery cell 2 and cooling fins.
Battery is typically constructed by rolling a sandwich of anode/separator/cathode in a sheet form into a jelly roll for cylindrical cells with superior thermal conduction in the same direction as the conductive anode/cathode sheets. It is due to the fact that anode/cathode sheets are constructed with metal with good thermal and electrical conductivity. The positive and negative connections are brought out in either the top plane 21 or bottom plane 22 in the same direction as the jelly roll. In a direction perpendicular to the sheets, thermal conduction is significantly worse, because heat must traverse metal, non-conductive separator, metal, non-conductive separator, several times before reaching the outer edge. For prismatic battery cells, it is typically constructed by a flattened version of a jelly roll or an interleaved anode/separator/cathode structure that also exhibit the same characteristics in thermal conduction. For pouch cells, construction is similar to prismatic battery cells except the outer enclosure is a soft pouch.
In other words, even though the distance between the top surface and the bottom surface is greater than the one between the opposite sides, thermal conductivity for the battery cell toward its top face and bottom surface is more efficient than toward the lateral sidewall. A factor of 12 or more in thermal conductance difference between top/bottom surface and sidewall is found in 18650 form factor battery cells. Consequently, prior art cooling fins or forced convection through the sidewall of a battery is hindered by the poor thermal conductivity of the sidewall and not effective in heat removal of the battery cell.